I just got a link removal request for links from not very successful (candidate for shutdown - one I definitely decide against trying to revive it) Q & A forum.
The links were to spam posts that had been removed, but remained accessible on a page showing past revisions. Of course my eye was off the ball in not having spotted this problem myself.
The reason they sent the request is that they have received an unnatural links penalty, and serve them right too - unless they are victims of negative SEO, of course.